This application claims priority under 35 USC 119 from Japanese patent application no. 09-336955, filed Dec. 8, 1997 and Japanese patent application no. 10-226101, filed Aug. 10, 1998.
1. Field of the Invention
The present invention pertains to a solid-state imaging apparatus for motion detection which detects differences between frames. Specifically, the present invention pertains to a solid-state imaging apparatus for motion detection that reduces external processing circuits and reduces erroneous detection of motion due to small background motions and noise when detecting motion.
2. Description of the Related Art
Prior art processing apparatuses for motion detection sequentially take images of image data from a solid-state imaging apparatus and detect motion based on differences between frames of this image data.
FIG. 9 is representative of a prior art image processing apparatus for motion detection 100. The image processing apparatus for motion detection 100 consists of a solid-state imaging apparatus 101, A/D conversion circuit 102 that converts the image signal (an analog signal) from the solid-state imaging apparatus 101 into a digital signal, image memory 103 (first image memory) and image memory 104 (second image memory) that save the digital signal from A/D conversion circuit 102, and an image processing circuit 105 that compares the digital image data saved in the image memories 103 and 104 against one another and detects motion.
In this image processing apparatus for motion detection 100, a first frame""s image signal (analog signal) obtained by solid-state imaging apparatus 101 is converted into a digital signal by the A/D conversion circuit 102, and then is saved in first image memory 103.
Next a second frame""s image signal (analog signal) obtained by solid-state imaging apparatus 101 is converted into a digital signal by the A/D conversion circuit 102, and then is saved in second image memory 104.
The image processing circuit 105 then compares pixels of the digital signal saved in the first image memory 103 with pixels of the digital signal saved in the second image memory 104. The processing circuit detects pixels that differ by more than a specified threshold value and generates a signal indicating detection of a moving object (hereafter the xe2x80x9cmoving object signalxe2x80x9d).
In this manner, inter-frame comparison makes it possible to detect motion of a subject.
Nevertheless, the aforesaid conventional image processing apparatus for motion detection 100 has defects in that the motion detection circuitry for the solid-state imaging apparatus 101 is complicated making the image processing apparatus for motion detection 100 overly large and costly.
Another defect is that the image signal output from solid-state imaging apparatus 101 is an analog signal, which is supplied to A/D conversion circuit 102 as an analog signal. Therefore, the analog signal is conducted along a path presenting an opportunity to be easily affected by noise (interference).
Furthermore, in the aforesaid conventional image processing apparatus for motion detection 100 the dynamic range of the analog image signal is limited by A/D conversion circuit 102. The input dynamic range of A/D conversion circuit 102 is usually narrower than the dynamic range of the solid-state imaging apparatus 101. Therefore, there is a defect in that the wide dynamic range of solid-state imaging apparatus 101 cannot be effectively used in the course of detecting and processing a moving object.
Also, the sampling timing in the A/D conversion circuit 102 may become slightly out-of-phase between the successive frames. This type of phase shifting in inter-frame sampling timing creates a slight phase shift in the pixel position to be compared at the image processing circuit 105. If this type of phase shift occurs, a stationary body may have inter-frame differences at its edge portions. Therefore, there is a defect in prior art solid-state imaging apparatuses that the precision and reliability of moving object detection are lowered.
One proposal for avoiding the aforesaid defects is to provide a memory to store the image signal for the immediately previous frame and the current frame in each pixel of solid-state imaging apparatus 101, and to additionally provide each pixel with a comparison circuit to compare the image signal stored in this memory, and to generate a moving object signal for each pixel.
However, this sort of countermeasure has the defects of making the structure of the unit pixel complicated, and reducing the numerical aperture and resolution of solid-state imaging apparatus 101. In addition, the aforesaid countermeasure can output only the moving object signal from each pixel. Thus, this design has the defect that the solid-state imaging apparatus would not be able to simultaneously provide an image signal, which is an undesirable result.
Incidentally, it is generally known that a solid-state imaging apparatus consisting of a semiconductor device experiences charge fluctuations, which create shot noise.
The magnitude of shot noise is proportional to the square root of the signal magnitude. Therefore, the brighter the subject and the higher the signal level, the greater the shot noise that is created. As a result, in bright locations shot noise looms large in inter-frame differences. If shot noise occurs in inter-frame differences and exceeds the threshold value for a moving-object decision, erroneous motion detection may occur.
One proposal for avoiding erroneous detection due to shot noise is to set the comparison threshold value for differences between frames uniformly high. Nevertheless, this sort of countermeasure has the problem that sufficient motion detection cannot be performed for a low-contrast subject.
Also, in addition to the case described above, differences between frames also occur in situations, such as when tree leaves wave in a wind. This sort of motion is small motion in the background, and should be distinguished from motion of the detection subject which is being monitored.
The present invention provides a solid-state imaging apparatus for motion detection which reduces external image comparison processing circuits and does not detect shot noise or small background motions when detecting motion. In addition, the present invention provides a solid-state imaging apparatus for motion detection which can simultaneously output a moving object signal and an image signal. Also, the present invention provides a solid-state imaging apparatus for motion detection which can selectively reduce erroneous detection of motion due to shot noise.
Furthermore, the present invention provides a solid-state imaging apparatus for motion detection which can reduce erroneous detection of motion in the screen""s horizontal direction and in the screen""s vertical direction. In addition, the present invention provides a solid-state imaging apparatus for motion detection which can reduce erroneous detection of motion in the time axis direction.
In a preferred embodiment of the present invention a solid-state imaging apparatus for motion detection includes a plurality of photoreceptive units arranged in a matrix and creating pixel output corresponding to incident light and a plurality of vertical read lines disposed on each column of the plurality of photoreceptive units. A vertical transfer circuit sequentially selects a specified row of the plurality of photoreceptive units and successively outputs to the vertical read line the previous frame""s pixel output saved in the past from the specified row of photoreceptive units, then the current frame""s pixel output newly saved from the specified row of photoreceptive units. A comparison circuit, disposed on each of the vertical read lines, compares the previous frame""s pixel output and the current frame""s pixel output transferred by time division via the vertical read lines. A horizontal transfer circuit horizontally transfers the aforesaid comparison circuit""s comparison results output on each of the vertical read lines. And, a logical calculation circuit performs a logical calculation on the comparison circuit""s comparison results and reduces the isolated regions of logical change.
The solid-state imaging apparatus for motion detection uses a vertical transfer circuit to output the xe2x80x9cprevious frame""s electrical signalxe2x80x9d and the xe2x80x9ccurrent frame""s electrical signalxe2x80x9d on the vertical read line by time division in row units. The comparison circuit takes the xe2x80x9cprevious frame""s electrical signalxe2x80x9d and the xe2x80x9ccurrent frame""s electrical signalxe2x80x9d output in this sort of time division manner and compares them. The horizontal transfer circuit horizontally transfers this comparison result.
The logical calculation circuit performs a logical calculation to reduce the isolated regions of logical change for this comparison result. In general, differences between frames arising from shot noise or tiny background motions occur randomly and briefly. Therefore, most of the noise component appears as isolated regions of logical change (in many cases, an isolated point) in the inter-frame comparison result.
In contrast, the detection subject occupies a greater area of the screen, and has a definite movement. The detection subject movement appears as band-like regions along the edge portion in the inter-frame comparison result. Therefore, by performing a logical calculation to reduce the isolated regions of logical change, the logical calculation circuit can efficiently reduce erroneous detection of motion caused by shot noise or tiny background motions.
The term xe2x80x9cframexe2x80x9d in this application means one frame""s worth of image. Accordingly, the solid-state imaging apparatus for motion detection need not be limited to devices which perform progressive scanning. For example, the present invention may be applied to devices that perform interlaced scanning. In interlaced scanning, detection of motion is based on the difference between the current field and the previous field.
The present invention also provides an image signal output circuit that selectively takes and horizontally transfers either the previous frame""s pixel output or the current frame""s pixel output transferred by time division via a vertical read line. This circuit selectively outputs one of the pixel outputs transferred by time division on a vertical read line, so the output circuit can output the current frame""s, or the previous frame""s, image signal.
In the present invention, the image signal output operation does not monopolize the vertical read line, so it does not interfere with the operation of the motion detection side. Therefore, it is possible to output a moving object signal and an image signal simultaneously.
The present invention also provides a level decision circuit that decides the level of an image signal output from the image signal output circuit, and an output switching circuit that switches and outputs the output of the logical calculation circuit and the comparison circuit""s comparison results according to the level decision circuit""s decision result.
Because shot noise occurs in proportion to the square root of the signal level, it is concentrated in the high luminance areas of the image signal. Therefore, by deciding whether the image signal exceeds a prescribed level, the level decision circuit can determine the regions most likely to include significant shot noise.
Therefore, for example, when the image signal exceeds a prescribed level the output switching circuit should selectively output the logical calculation circuit output, and when the image signal falls below a prescribed level the output switching circuit should output the comparison circuit""s comparison result. This sort of switching operation can selectively and reliably reduce erroneous detection of motion caused by shot noise. Additionally, there is no unnecessary removal of isolated regions with little shot noise, so the detection apparatus can reliably detect the motion of small detection subjects.
On the other hand, in cases in which the signal level is extremely small, random noise from circuit systems and the like dominate. (In particular, random noise is amplified and strongly manifested in connection with signal level drops when a peak AGC circuit or the like is interposed in a circuit.) Performing output switching between xe2x80x9clogical calculation circuit outputxe2x80x9d and xe2x80x9ccomparison circuit""s comparison resultxe2x80x9d in response to the level decision circuit""s decision result as in the examples given above makes it possible to selectively reduce motion detection errors due to noise, etc., and makes it possible to detect the motion of small detection subjects.
The present invention also provides a solid-state imaging apparatus for motion detection having a bit memory circuit that stores the comparison result for each 1 pixel horizontally transferred, and a horizontal AND circuit that performs a logical AND calculation on the bit memory circuit""s stored contents and the comparison result.
The present invention also provides a solid-state imaging apparatus for motion detection having a line memory circuit that stores the comparison result for each 1 line, and a vertical AND circuit that performs a logical AND calculation on the line memory circuit""s stored contents and the comparison result.
The present invention also provides a solid-state imaging apparatus for motion detection having a frame memory circuit that stores the comparison result for each 1 screen, and a time axis AND circuit that performs a logical AND calculation on the frame memory circuit""s stored contents and the comparison result.
In preferred embodiments, the present invention also provides a solid-state imaging apparatus having a pixel output save unit, provided for each of the photoreceptive units, which saves the pixel output from the photoreceptive unit, and nondestructively outputs the saved pixel output, a connection and separation unit, provided for each of the pixel output save units, which connects and separates the pixel output save unit""s output stage and the vertical read line, and vertical transfer control means which, after the previous frame""s pixel output saved in the past at the specified row""s pixel output save unit is output to the vertical read line via the connection and separation unit, newly saves the current frame""s pixel output from the photoreceptive units in the pixel output save unit, and outputs the saved current frame""s pixel output to the vertical read line via the connection and separation unit.
The present invention also provides a solid-state imaging apparatus having an amplifier element which has a control region that saves pixel output and that outputs a signal corresponding to the pixel output saved in the control region, a transfer circuit that transfers the pixel output created by the photoreceptive unit to the amplifier element""s control region, and a reset circuit which resets the pixel output accumulated in the amplifier element""s control region.
The present invention also provides a solid-state imaging apparatus for motion detection wherein the amplifier element is a junction type field-effect transistor, and the pixel output transferred via the transfer circuit is directly accumulated in the gate region of the junction type field-effect transistor.
The present invention also provides a solid-state imaging apparatus for motion detection wherein the comparison circuit is a circuit that decides whether or not the current frame""s pixel output and the previous frame""s pixel output agree within an allowed range, and that outputs a binary signal according to the truth or falsity of the decision result.